I visited the place where the mystery rock came from and I thought it came out of a pond out in a farm field. It came from a shale bed that had a pond on it. So it looks more and more likely the rock was a marcasite nodule. This is iron sulfide or a form of pyrite. I am not sure how to tell the crystal structure so maybe it is pyrite or marcasite or some mixture.
Here are some pictures of the New Albany shale layer dug up in Memphis Indiana.